[gcc/devel/c++-modules] libstdc++: Also disable caching of reverse_view::begin() for common_ranges

[Nathan Sidwell]

https://gcc.gnu.org/g:77e596cf3c1b7bc11ba946394ed8d62a49157b49

commit 77e596cf3c1b7bc11ba946394ed8d62a49157b49
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Feb 28 10:47:26 2020 -0500

    libstdc++: Also disable caching of reverse_view::begin() for common_ranges
    
    When the underlying range models common_range, then reverse_view::begin() is
    already O(1) without caching.  So we should disable the cache in this case too.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (reverse_view::_S_needs_cached_begin): Set to false
            whenever the underlying range models common_range.

Diff:
---
 libstdc++-v3/ChangeLog          | 3 +++
 libstdc++-v3/include/std/ranges | 3 ++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 89e1f5bf952..0feef87cce6 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,8 @@
 2020-02-28  Patrick Palka  <ppalka@redhat.com>
 
+	* include/std/ranges (reverse_view::_S_needs_cached_begin): Set to false
+	whenever the underlying range models common_range.
+
 	* include/std/ranges (__detail::_CachedPosition): New struct.
 	(views::filter_view::_S_needs_cached_begin): New member variable.
 	(views::filter_view::_M_cached_begin): New member variable.
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 2f773130979..19d3da950e7 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3177,7 +3177,8 @@ namespace views
     private:
       _Vp _M_base = _Vp();
 
-      static constexpr bool _S_needs_cached_begin = !random_access_range<_Vp>;
+      static constexpr bool _S_needs_cached_begin
+	= !common_range<_Vp> && !random_access_range<_Vp>;
       [[no_unique_address]]
 	__detail::__maybe_empty_t<_S_needs_cached_begin,
 				  __detail::_CachedPosition<_Vp>> _M_cached_begin;